Passive liquid crystal displays (LCDs) have become ubiquitous in today's display market, often incorporated into digital watches, calculators, etc. There are two primary types of passive liquid crystal displays, a segmented display having sections of liquid crystal material that can be arranged into a template or pattern which can form any numeral (and virtually any letter), and a passive matrix display in a row-column format where picture elements or pixels of liquid crystal material are located at intersections between the rows and columns.
These passive liquid crystal displays arrange liquid crystal material having a helical structure between a pair of glass panels embedded with electrodes, which are then enclosed by a pair of polarization filters. The helical structure of the liquid crystal material bends or rotates light entering the liquid crystal display through one of the polarization filters, such that it can substantially propagate through the other polarization filter, causing that section of the display to appear light gray to a user. When voltage is supplied to the electrodes of a section, however, the difference in the voltage causes liquid crystal molecules to begin to untwist from their helical structure and attempt to align with the electric field. When the voltage difference is large enough, the liquid crystal molecules become substantially untwisted and thus allow light to pass through unrotated, causing the polarizing filters to block the light from reaching the user, or appearing black.
FIG. 1A shows a conventional display driver 100. Referring to FIG. 1A, the conventional display driver 100 includes a voltage generator 110 to generate bias voltages V0, V1, and V2 and provide the bias voltages V0-V2 to a plurality of common drivers 120-1 to 120-N and a plurality of segment drivers 130-1 to 130-M. The common drivers 120-1 to 120-N and segment drivers 130-1 to 130-M provide one of the bias voltages V0-V2 and to a liquid crystal display (not shown) via output pins COM0-COMn and SEG0-SEGm, respectively. Each common pin-segment pin pair corresponds to a section of liquid crystal material that is untwisted (or not) according to the difference between selected voltages applied to the common pin-segment pin pair. For instance, a liquid crystal section corresponding to pins COM0 and SEG3 will untwist or not according to the difference in the voltage applied to the pins COM0 and SEG3, while a liquid crystal section corresponding to pins COM0 and SEGm will untwist or not according to the difference in the voltage applied to the pins COM0 and SEGm.
To avoid damaging the liquid crystal material during operation, the voltage applied to the electrodes or output pins must be alternated, typically with the common drivers 120-1 to 120-N and segment drivers 130-1 to 130-M selecting different voltages multiple times per frame. As such, a voltage response in a transient period of the voltage switching can reduce the voltage being applied to the output pins, thus lowering the voltage difference and affecting the color or gradation of gray displayed by the corresponding section in the LCD panel.
FIG. 1B shows a diagram of voltage switching in a conventional display driver 100. Referring to FIG. 1B, the diagram is a volts-versus-time graph showing a voltage waveform provided through at least one of the common drivers 120-1 to 120-N or segment drivers 130-1 to 130-M when switched between voltages V2 and V0. After each voltage switch, there is a transient period where the voltage approaches a steady state of V2 or V0.
When the actual voltage response is less than the ideal voltage response, the voltage difference perceived in the LCD panel is lower than expected. This reduced voltage difference causes liquid crystal molecules in the corresponding section(s) to untwist less, thus allowing light to propagate through the polarization filter to the user. In other words., conventionally the LCD panel section may not become black enough during this transient period.
To help increase the voltage response, the conventional display driver 100 utilizes low impendence switches in the common and segment drivers 120-1 to 120-N and 130-1 to 130-M, respectively. The low impedance switches can increase current to flow to the LCD panel, and thus increase the voltage response, bringing it closer to the ideal response shown in FIG. 1B. The conventional display driver 100 can also include several capacitors 101-103 corresponding to each bias voltage V0-V2 to improve the voltage response in the LCD panel. Low impendence switches and capacitors 101-103, however, consume a large amount of area on integrated circuits, which significantly increases the size and cost of the conventional display driver 100.